This invention relates to a multilayer ceramic substrate or body having two adjacent layers of different composition and dielectric properties such as disparate dielectric constants.
Such bodies are typically used as substrates that include buried conductors and buried capacitors. Some employ conductive vias for making electrical interconnection between layers of conductors. It is also known to metallize portions of an outer surface of such bodies and to mount discrete electrical components thereto, e.g. semiconductor integrated circuits, and/or forming on the body and in it film components such as resistors. It is desirable to form such buried circuits and surface components in and on a low dielectric constant material to minimize interwiring capacitance and consequent "cross talk". On the other hand, it is often highly desirable to form buried capacitors within a high dielectric constant body to minimize the physical size and cost of the capacitor.
The above-noted low K materials when combined in the green state with a layer or layers of barium titanate and sintered to a dense mature compound body, exhibit at each interface with the barium titanate, a band of an integrated material wherein some of the elements of each of the two start materials have co-reacted. This interface band generally exhibits a gradient of dielectric properties, e.g. dielectric constant, DF, and TC, that are quite different than that of either start material, i.e. either of the low K material or the high K barium titanate layer. This band cannot be used as a capacitor dielectric in practice because of its varying and uncertain dielectric properties. The thickness of this interface band of co-reacted material in compound prior art bodies is typically 0.025 to 0.065 mm (1.0 to 2.6 mils). It is good practice to relegate even wider bands of material at such interfaces to non-use and safely avoid unexpected and degraded performance of capacitors formed therein.
Also, the compound ceramic bodies of the prior art that combined a high dielectric constant barium tianate layer with a low dielectric constant layer have a strong tendency to develop catastrophic cracks due to large differences in the thermal coefficients of expansion of these two dissimilar materials. This severely limits the overall compound-body size that can be reliably manufactured.
Other prior art devices have a high dielectric layer sandwiched between two layers of a low dielectric ceramic composition, in which the outer layers are made as thick or thicker than the high dielectric constant layer to ameliorate the tendency of these compound bodies for cracking. Such a limitation on sandwiched ceramic bodies more often leads to thicker and larger packages than desirable and than would otherwise be necessary.
It is an object of the present invention to provide a low dielectric constant ceramic material that may be combined in a composite body with and reliably co-sintered with a barium titanate.
It is a further object of this invention that said ceramic material be chemically and physically compatible at co-sintering with barium titanate.
It is another object of the present invention to ameliorate the above-noted shortcomings of the prior art.
It is a further object of this invention to form a multilayer compound ceramic body having an inner layer of barium titanate sandwiched between two layers of said ceramic material having a dielectric constant at least 100 times less than that of the barium titanate.